A motor vehicle headlamp essentially comprises a lamp which is located at the focal centre of a concave reflector so as to emit a particular light beam satisfying safety requirements. The reflector has to have a reflective surface of particularly high quality, so as to ensure that the reflected beam is properly distributed, without faults or irregularities which might give rise to parasitic reflections that could cause dazzle. At the same time, for reasons of cost and in order that the reflector can be made in shapes which are becoming more and more complex, these reflectors are currently made of plastics material.
Thermoplastic materials facilitate moulding which is easily controllable, and are found to be generally satisfactory in that their surface quality is compatible with the required optical function. However, these thermoplastic materials have the drawback that they are sensitive to heating caused by the lamp itself when the headlamp is illuminated. This disadvantage manifests itself in the appearance of local deformations which affect the quality of the reflected light beam. The smaller the volume of the reflector, the more apparent is this undesirable effect.
Better resistance to heating can be obtained from thermosetting plastics materials. The compositions which are used essentially comprise: a resin with an unsaturated polyester base; an agent for preventing shrinkage, such as a thermoplastic composition; and glass fibres which act as reinforcing material.
The appropriate proportions of the various constituents have been determined experimentally in such a way that a surface quality free of asperities or fissures (i.e. a surface of optical quality) is able to be made by injection moulding so that the reflectors are suitable for use in motor vehicle headlamps. It has been established that any departure, however small, from these appropriate dimensions is detrimental to the properties of the material of the reflector, as regards the surface quality and/or mechanical strength and chemical and mechanical stability of the reflector.
However, even when using the appropriate proportions of the various constituents, it has been noticed that moulding of small reflectors does not give satisfactory results. In particular, these small reflectors tend to have very prominent local angular profiles in certain places. Various moulding faults have been found in the parts of the reflector where these angular profiles occur. These faults include, for example, heterogeneous distribution of the reinforcing fibres: this gives rise to mechanical weakness under the effect of mechanical and/or thermal shock.